Metallic thin film and method of manufacturing the same, and surface acoustic wave device using the method thereof

ABSTRACT

By using a dual ion-beam sputtering apparatus, an aluminum thin-film is formed on a glass substrate made of an amorphous material. While radiating an ion beam for assisting the film formation from an ion source onto the glass substrate, the aluminum thin-film is formed by depositing the sputtering ions which are generated by radiating an ion beam onto an aluminum target.

This is a divisional of application Ser. No. 08/786,410, filed Jan. 21,1997, now U.S. Pat. No. 6,033,471.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metallic thin-film applied toelectrodes of various kinds of electronic parts and also relates to amethod of manufacturing the metallic thin-film.

2. Description of the Related Art

As a result of the recent advances, the size of various types ofelectronic components such as capacitors, LSIs, coils, filters andoscillators have been reduced. This has made it necessary to providethin-film electrodes for these components (such films are typicallyformed of aluminum or the like) which exhibit increased resistance toelectric power. The crystallinity of such thin-films largely affectstheir resistance to electric power. When the thin-film material ispolycrystalline or amorphous, atoms diffuse within grain boundariesunder a high current density or a high stress which forms voids orhillocks in the thin-film. These voids or hillocks cause short circuitsor disconnection. This phenomenon is referred to as electromigration orstressmigration.

To avoid this problem, it is necessary to create mono-crystallinestructures which do not have the grain boundaries which serve asdiffusion paths for the atoms. However, it is impossible to create sucha mono-crystalline structure on a surface of an amorphous body, apolycrystalline body, or an orientation layer of a polycrystalline bodyby conventional film-forming methods.

SUMMARY OF THE INVENTION

The process of the present invention provides a metallic thin-film whichis entirely or partially mono-crystalline in structure formed on thesurface of a support body which has an amorphous or a polycrystallinestructure, or which has a polycrystalline orientation layer, and amethod for manufacturing such metallic thin-films.

In the preferred embodiment, the thin film electrode is arranged for usein a surface acoustic wave device which better withstands electric powerthan the prior art devices.

In accordance with the preferred embodiment, a metallic thin-film of thepresent invention has an entirely or partially mono-crystallinestructure, and is formed on the surface of an amorphous body, apolycrystalline body, or an orientation layer of a polycrystalline body,under the influence of a radiated beam which assists the film formationonto the surface of the amorphous body, the polycrystalline body, or theorientation layer.

Preferably, at least one type of ion selected from the group consistingof argon, helium, neon, krypton, and xenon ions, or a mixture thereof isused for the irradiating ion beam. The metallic thin-film is preferablycomposed of aluminum, gold, silver, copper, platinum, titanium,chromium, nickel, tungsten, an alloy thereof, or an alloy essentiallyconsisting of at least one of the above elements.

According to one aspect of the present invention, there is provided athin film electrode for use in a surface acoustic wave device and asurface acoustic wave device comprising an amorphous layer formed on apiezoelectric substrate, and at least one of a single crystal layer andan oriented layer formed on the amorphous layer.

According to another aspect of the present invention, there is provideda method of forming a surface acoustic wave device having a thin filmelectrode formed thereon, the method comprising the steps of forming anamorphous layer on a surface of a piezoelectric substrate whileirradiating the surface of the piezoelectric substrate with an ion beam,and thereafter forming at least one of a single crystal layer and anoriented layer on a surface of the amorphous layer with the assist ionbeam.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawing a plurality of forms which are presently preferred, it beingunderstood, however, that the invention is not limited to the precisearrangement and instrumentalities shown.

FIG. 1 is a diagram of an apparatus for manufacturing metallicthin-films, which apparatus illustrates an embodiment of a metallicthin-film and a method of manufacturing the film incorporated in thepresent invention; and

FIG. 2 is a sectional diagram of a single-crystallized metallicthin-film formed on the surface of an amorphous body.

FIG. 3 is a cross-sectional view of the thin film electrode, including asingle crystal structure or an orientation layer structure, which isformed on the surface of the piezoelectric substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(First Embodiment)

The present invention will be better understood from the followingdescription of the preferred embodiment of a metallic thin-film and amethod of manufacturing the film taken in conjunction with theaccompanying drawings. According to the a first embodiment of thepresent invention, a (111) orientated aluminum thin-film is formed on aglass substrate 12.

FIG. 1 shows a dual ion-beam sputtering apparatus 1 for forming a filmon the glass substrate 12. The dual ion-beam sputtering apparatus 1includes a vacuum chamber 2 and a vacuum pump (not shown in the figure)connected to an exhaust port 2 a provided in the right wall of thevacuum chamber 2. Inside the vacuum chamber 2, an ion source 3 forsputtering and an ion source 4 for assisting the film formation arepositioned on the left side, and a backing plate 5 and a substratesupport 6, which is located above the backing plate 5 are positioned onthe right side.

A thin-film is deposited using the sputtering apparatus 1 as follows. Analuminum target 11, which acts as a sputtering material, is fixed to thesurface of the backing plate 5 using a brazing filler metal such asindium. A glass substrate 12 (i.e., of an amorphous material) is fixedonto the surface of the substrate support 6. Air in the vacuum chamber 2is evacuated to preferably 0.1 Pa or less from the exhaust port 2 ausing the vacuum pump. If the vacuum exceeds 0.1 Pa, the resultingthin-film will contain residual gases, such as H₂O. In this embodiment,the vacuum chamber 2 is maintained at 5×10⁻³ Pa.

A so-called “ion assisting” process is conducted by radiating an ionbeam 21 which assists the formation of a thin film onto the surface ofthe glass substrate 12, and is continued until the completion of theformation of the film. The preferred energy for the ion beam 21 is from100 eV to 1 KeV. If the energy is less than 100 eV, sufficient energycannot be provided for the atoms in the thin-film 13, while energy above1 KeV is exceedingly high so that the ion beam 21 disadvantageouslysputters and disperses the atoms in the thin-film 13 deposited on thesurface of the glass substrate 12, thus preventing the thin-film fromgrowing.

The preferred current density of the ion beam 21 is 0.01 to 20 mA/cm².If the current density is less than 0.01 mA/cm², sufficient energycannot be provided for the atoms in the thin-film 13, while a currentdensity above 20 mA/cm² is exceedingly high so that the ion beam 21disadvantageously sputters and disperses the atoms in the thin-film 13deposited on the surface of the glass substrate 12, thus preventing thethin-film from growing. The incident angle of the ion beam 21 to theglass substrate 12 is preferably set within a range of from 0° to 45°with respect to a line which is normal to the surface of the glasssubstrate 12. If the incident angle does not fall within the above anglerange, it becomes difficult to efficiently provide energy for the atomsin the thin-film 13.

An argon ion-beam is preferably radiated onto the aluminum target 11from the ion source 3 for sputtering. The argon ions collide with thealuminum target 11 to disperse ions 23 therefrom. These ions 23 reachthe surface of the glass substrate 12 and are deposited thereon to forman aluminum thin-film 13. The film-forming speed is preferably 0.001nm/sec or more. When the speed is less than 0.001 nm/sec, the atoms inthe thin-film 13 aggregate, resulting in grain growth. In the presentembodiment, film-forming is conducted at 0.06 nm/sec and at 25° C. Sincethe glass substrate 12 is made of an amorphous material, the aluminumthin-film 13 does not have a mono-crystalline atomic structure in theinitial stage of the film formation. However, the influence of the glasssubstrate 12 disappears and the thin-film 13 can attain amono-crystalline atomic structure when several atomic layers have beendeposited. After film-forming, the glass substrate 12 is removed fromthe vacuum chamber 2. The surface of the aluminum thin-film 13 depositedon the surface of the substrate 12 may form an oxidized layer 13 a bycontact with an oxygen atmosphere, as is shown in FIG. 2.

The resulting aluminum thin-film 13 was confirmed to have amono-crystalline structure based on evaluation according to the Rheedmethod. Therefore, the grain-boundary diffusion of atoms under highcurrent-density or high stress is preventable by employing the aluminumthin-film 13, resulting in a reliable electrode. For example, theresistance of the aluminum thin-film 13 to electric power isapproximately 100 times that of a polycrystalline aluminum thin-film.

The aluminum thin-film 13 has a small degree of lattice defects andexhibits excellent corrosion resistance such that the number of corrodedportions decreases to approximately one tenth that of a polycrystallinealuminum thin-film. Thus, in addition to the usage as an electrodematerial, the thin-film of the present invention can be advantageouslyemployed as, for example, a thin-film material for ornamental articlessuch as mirrors and a thin-film material used in a chloride atmospheresuch as on the seashore or in the sea.

A metallic thin-film and a method of manufacturing the film incorporatedin the present invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresent embodiment is therefore to be considered in all respects asillustrative and not restrictive.

Besides the argon ions, helium, neon, krypton, or xenon ions may beemployed alone or in combination for the ion beam 21 for assisting thefilm formation. Gold, silver, copper, platinum, titanium, chromium,nickel, tungsten, an alloy thereof, or an alloy essentially consistingof at least one of the above elements may be employed as the thin-filmmaterial, in addition to aluminum. Additionally, the sputteringconditions vary according to the combination of the thin-film materialand an amorphous body or the thin-film material and a polycrystallinebody or an orientation layer of a polycrystalline body.

Although a (111) oriented aluminum thin-film is formed on a glasssubstrate in the above embodiment, the orientation direction of thethin-film is optional and, for example, a (200) oriented aluminumthin-film may be formed. A boron nitride substrate or the like may beused besides the glass substrate. Further, the thin-film of the presentinvention may be formed on a thin-film instead of the substrate. It isnot necessary that the substrate on which the thin-film is formed isentirely amorphous, as long as the surface thereof is amorphous.Moreover, the substrate may be a polycrystalline body or comprise anorientation layer. As a film-forming method, evaporation, chemical vapordeposition, molecular beam epitaxy, laser ablation, or the like may beemployed in addition to sputtering. Furthermore, the metallic thin-filmis not required to be entirely mono-crystalline in structure, as long asit is partially mono-crystalline in structure.

(Second Embodiment)

A thin film electrode for use in a surface acoustic wave device, asurface acoustic wave device, and a method of forming the thin filmelectrode and the surface acoustic wave device which represent anembodiment of the present invention will be described. The embodimentwill be performed with respect to a case of forming a (111)-orientedaluminum thin film on a lithium tantalate substrate in the same methodand conditions as in the first embodiment.

FIG. 3 shows a cross-sectional view of a thin film electrode 24 of asurface acoustic wave device according to the present invention. Thethin film electrode 24 is formed, for example, as a planar comblike IDT(interdigital transducer) electrode by sputtering on the substrate onwhich masking was performed or by etching the thin film electrode whichwas formed on the entire surface of the substrate.

At an initial stage of the formation of aluminum thin film 24, under theinfluence of the ion assist, thin film 24 forms an amorphous layer 24 aas shown in FIG. 3. As the film formation progresses further, thin film24 forms a mono-crystalline layer (or highly-oriented layer) 24 b. Afterthe thin film formation, when the substrate 23 having aluminum thin film24 formed on its substrate is taken out of the vacuum chamber 2, anoxide layer 24 c is formed in the surface of the aluminum thin film 24brought in contact with air.

As described above, according to the present invention, an entirely orpartially mono-crystalline metallic thin-film is formed on the surfaceof an amorphous body, a polycrystalline body, or an orientation layer ofa polycrystalline body, while radiating an ion beam for assisting thefilm formation onto the surface of the amorphous body, thepolycrystalline body, or the orientation layer. Since the resultingmetallic thin-film contains a mono-crystalline structure, thegrain-boundary diffusion of atoms under high current density or highstress is preventable. Furthermore, the metallic thin-film has a smalldegree of lattice defects and exhibits excellent corrosion resistance.Thus, a reliable electrode can be obtained by using the metallicthin-film.

What is claimed is:
 1. A surface acoustic wave device, comprising: apiezoelectric substrate; and an interdigital transducer made of a thinfilm and formed on said substrate, said thin film including an aluminumamorphous layer on said substrate and at least one of an aluminum singlecrystal layer and an aluminum oriented layer formed on said aluminumamorphous layer.
 2. A surface acoustic wave device according to claim 1,wherein said thin film further includes an oxide layer on said at leastone of an aluminum single crystal layer and an aluminum oriented layer.3. A surface acoustic wave device according to claim 1, wherein said atleast one of an aluminum single crystal layer and an aluminum orientedlayer has a (111) orientation.
 4. A surface acoustic wave deviceaccording to claim 1, wherein said piezoelectric substrate is made oflithium tantalate.
 5. A surface acoustic wave device according to claim1, wherein said thin film is formed by a sputtering method with an ionbeam assist.